Aberration correcting element and assembly

ABSTRACT

An aberration correcting element includes a first electrode layer including a plurality of electrode portions; a second electrode layer opposed to the first electrode layer; and a liquid crystal sandwiched between the first and second electrode layers for producing a phase change corresponding to voltages applied to the first and second electrode layers in a light beam passing therethrough. The first electrode layer includes a plurality of position markers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aberration correcting element and anaberration correcting assembly for correcting aberration of a lightbeam.

2. Description of the Related Art

Optical discs such as a CD (Compact Disc) and a DVD (Digital Video Discor Digital Versatile Disc) are well known as information recording mediafor optically recording and reproducing information. Furthermore, thereare various types of optical discs, for example, optical discs forreproduction only, write-once optical discs on which only additionalrecording can be done, and rewritable optical discs on which informationcan be erased and re-recorded.

Research and development are in progress for developing high-densityoptical discs and optical pickups and information recording and/orreproducing apparatuses (hereinafter referred to asrecording/reproducing apparatus) applicable to the high-density opticaldiscs. In addition, research and development have also been pursued fordeveloping optical pickups and information recording/reproducingapparatuses having the capability to be used for optical discs ofdifferent types.

A method of using with the high-density discs by increasing a numericalaperture (NA) of an objective lens provided in the pickup apparatus hasbeen considered. Another method is the use of a light beam having ashorter wavelength.

However, the aberration of the light beam caused by an optical disc isincreased as the numerical aperture NA of the objective lens isincreased or a light beam having a shorter wavelength is used. Thismakes it difficult to improve performance accuracy of therecording/reproduction of information.

For example, when an objective lens having a large numerical aperture isused, the amount of birefringence distribution, which depends on theincidence angle, is increased at the pupil surface of the optical disc,since the range of the incidence angle of the light beam to the opticaldisc is increased. This creates an aberration problem due to thebirefringence becoming more influential. In addition, the effect ofaberration produced by error in the thickness of a disc cover layer forprotecting the disc recording surface increases.

One example of such an aberration correcting element is disclosed, forexample, in Japanese Patent Application Kokai H10-269611. The aberrationcorrecting element has a plurality of coaxial electrodes made oftransparent material. The orientational ordering in a liquid crystal isadjusted by changing the voltage applied to each of the electrodes.Thus, the aberration produced in the light beam is corrected.

It is necessary to align the aberration correcting element preciselywith the objective lens in order to correct the aberration accurately.Accordingly, in some cases, the aberration correcting element ispreviously aligned with the objective lens and fixed to the lens to forman integrated aberration correcting assembly. The aberration correctingelement is aligned with the objective lens by adjusting relativepositions while applying a voltage to the transparent electrodes of theaberration correcting element to produce diffraction and observing animage through the objective lens with an imaging device such as a CCDcamera when manufacturing the aberration correcting assembly.

It is a problem in the above-described alignment method that thecontrast of the transparent electrode pattern is unclear and, therefore,the alignment accuracy is low. Furthermore, it is difficult to ascertainthe amount of positional deviation between the objective lens and theaberration correcting element when the positional deviation occurs.

OBJECT AND SUMMARY OF THE INVENTION

The present invention has been made to overcome the above-describedproblems, and it is an object of the present invention to provide anaberration correcting element capable of being accurately aligned withan optical element, as well as a high-performance aberration correctingassembly in which an optical element and an aberration correctingelement are aligned with high accuracy.

To achieve the object, according to one aspect of the present invention,there is provided an aberration correcting element which includes afirst electrode layer having a plurality of electrode portions, a secondelectrode layer opposed to the first electrode layer, and a liquidcrystal sandwiched between the first and second electrode layers forproducing a phase change corresponding to voltages applied to the firstand second electrode layers in a light beam passing therethrough,wherein the first electrode layer includes a plurality of positionmarkers.

According to another aspect of the present invention, there is providedan aberration correcting assembly, which comprises a first electrodelayer including a plurality of electrode portions and first positionmarkers; a second electrode layer opposed to the first electrode layer;a liquid crystal sandwiched between the first and second electrodelayers for producing a phase change in a light beam passing therethroughcorresponding to voltages applied to the first and second electrodelayers; and an optical element including second position markerscorresponding to the first position markers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of an optical pickuphaving an aberration correcting assembly according to an embodiment ofthe present invention;

FIG. 2 is a cross-sectional view schematically showing the structure ofthe aberration correcting assembly;

FIG. 3 is a perspective view schematically showing the structure of anaberration correcting element;

FIG. 4 is a plan view schematically showing the structure of a firstelectrode layer in an aberration correcting element for correctingspherical aberration;

FIG. 5 is a enlarged view of portion A shown in FIG. 4;

FIG. 6 is a view showing the arrangement of an aberration correctingelement, an objective lens, and an observational camera when adjustmentis performed;

FIG. 7 is a view showing one example of position marker;

FIGS. 8A and 8B are views showing other examples of position marker; and

FIG. 9 is a plan view schematically showing the structure of anelectrode layer of an aberration correcting element for correcting comaaberration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the attached figures, the preferred embodiments of thepresent invention will be described in detail. In the figures used inthe following description, substantially identical components areindicated by identical numerals.

FIG. 1 is a block diagram showing the structure of an optical pickup 10having an aberration correcting assembly 4 according to an embodiment ofthe present invention.

A light beam emitted from a laser light source 1 mounted in the opticalpickup 10 is collimated by a collimating lens 2. The collimated lightbeam is made incident on an optical disc 9 via a polarization beamsplitter 3 and the aberration correcting assembly 4. The incident lightbeam is reflected by the optical disc 9. The reflected light beam passesthrough the aberration correcting assembly 4. The light beam is, then,reflected by the beam splitter 3, collected by a collector lens 6, anddetected by an optical detector 7.

The aberration correcting assembly 4 includes an aberration correctingelement 41, a quarter wavelength plate 42, an aperture stop 43, and ahigh numerical aperture objective lens 45 comprising two lenses as shownin the cross-sectional view of FIG. 2. These optical elements are heldstationary by a microscope column 47. As described in detail later, theaberration correcting element 41 and the high numerical apertureobjective lens 45 are placed and fixed in position such that theiroptical axes are in alignment with each other.

The aberration correcting element 41 has an electro-optical device thatshows an electro-optical effect when an electric field is appliedthereto. The aberration correcting element 41 includes an opticalelement (e.g., a liquid crystal) that shows a variation in birefringenceaccording to the magnitude of an applied control voltage Vi. Moreparticularly, the aberration correcting element 41 includes a firstelectrode layer 17, a second electrode layer 18, and a liquid crystal 19sealed in and sandwiched between the two electrode layers asschematically shown in the perspective view of FIG. 3. Morespecifically, a liquid crystal orientation film 21, a transparentinsulating layer 23, and the first electrode layer 17 are successivelyformed on one surface of the liquid crystal 19. On the other surface,there are successively formed a liquid crystal orientation film 22, atransparent insulating layer 24, and the second electrode layer 18.Insulating substrates 15 and 16 such as transparent glass plates areformed on the first electrode layer 17 and the second electrode layer18, respectively.

At least one of the first electrode layer 17 and the second electrode 18has a plurality of electrode portions. The electrode pattern of theelectrode portions corresponds to a plurality of phase-adjustingportions. Aberration in the light beam passing through the aberrationcorrecting element 41 can be corrected by applying a voltagecorresponding to the profile of the distribution of the aberrationproduced in the light beam to each of the electrode portions. In theembodiment described below, the first electrode layer 17 is configuredto have a plurality of electrode portions.

The structure of the first electrode layer 17 of the aberrationcorrecting element 41 for correcting spherical aberration isschematically shown in the plan view of FIG. 4. The first electrodelayer 17 has a plurality of coaxial, annular phase adjusting portionscorresponding to the distribution of the spherical aberration producedin the light beam from the optical disc 9. As shown in the enlarged view(portion A of FIG. 4) of FIG. 5, narrow and annular metal electrodes Mi(i=1, 2, . . . ) and transparent electrodes Ei (i=1, 2, . . . )comprising a plurality of ITO (indium-tin oxide)films partitioned by themetal electrodes Mi are formed. The width of the metal electrodes Mi isset to several micrometers (μm), for example, to prevent the passinglight beam from being affected. The metal electrodes Mi are connected tolead electrodes for applying voltages to the metal electrodes Mi. Whencontrol voltages Vi (i=1, 2, . . . ) are applied to the metal electrodesMi via the lead electrodes, an electric field which varies graduallyaccording to the resistivity of the ITO films is applied to the liquidcrystal 19. Correspondingly, the orientation of liquid crystal moleculesin the various portions of the liquid crystal 19 varies. The lightpassing through the liquid crystal 19 varies in phase by undergoing thebirefringence of the liquid crystal 19. As a result, the phase of thelight can be controlled by the control voltages Vi applied to the liquidcrystal 19.

Three position markers 51 for alignment are formed at an outer peripheryof the region of the aberration correcting element 41 corresponding tothe aperture diameter (pupil diameter) of the objective lens 45. In thecase shown in FIG. 4, the three position markers 51 are identical inshape and equally spaced from each other by 120 degrees, and areequidistant from the center of the optical axis. Positional deviationwithin a plane can be determined by providing the position markers 51 atthree predetermined positions that are not on a straight line.

FIG. 6 is a view showing the arrangement of the aberration correctingelement 41, the objective lens 45, and a camera for observation (e.g., aCCD camera) when positioning or adjustment of the aberration correctingelement 41 and the objective lens 45 is performed. Light from a lightsource for adjustment is passed through the aberration correctingelement 41 and the objective lens 45. The relative position of theaberration correcting element 41 and the objective lens 45 is adjustedand fixed while observing the light passed therethrough with the CCDcamera 55. The CCD camera 55 is connected to a control unit (not shown)such as a microcomputer for image processing or position adjustment.

Each of the position markers 51 is shaped to have an array of strips 52each having a length L and a width W as shown in FIG. 7. The distance Dbetween the strips 52 is set to a desired value (e.g., 10 μm). In thisway, the markers can be used as a scale for measuring the amount ofpositional deviation. Therefore, the amount of relative positionaldeviation between the aberration correcting element 41 and the objectivelens 45 can be accurately measured.

The manufacturing process can be simplified by forming the positionmarkers 51 from the same material as the metal electrodes of theaberration correcting element 41. The position markers 51 are not alwaysrequired to be identical in shape. For example, as shown in FIGS. 8A and8B, a circular- or crisscross-shaped position marker can be formed inthe center of the aberration correcting element 41, thus permittingaccurate alignment with the center of the objective lens 45. Theposition markers 51 may be visually observable markers and can be madeof various materials. Furthermore, various methods can be employed. Forinstance, the position markers may be printed on the metal electrodes ofthe aberration correcting element 41 using printing techniques.

In the description given above, the aberration correcting assembly 4 inwhich the aberration correcting element 41 for correcting sphericalaberration and the objective lens 45 are connected together is given asan example. The invention can also be applied to the aberrationcorrecting element 41 for correcting various aberrations such as comaaberration and astigmatism. For example, FIG. 9 is a plan viewschematically showing the structure of the first electrode layer 17 ofthe aberration correcting element 41 for correcting coma aberration.

The first electrode layer 17 is formed in a pattern of electrodescorresponding to the distribution of coma aberration to be corrected.More particularly, the layer is symmetric with respect to the axispassing through the center point. Three perpendicular crisscrossposition markers 51A are formed at the outer periphery of the region ofthe aberration correcting element 41 corresponding to the aperturediameter of the objective lens 45. A position marker 51B is formed inthe center. The diameter of the position marker 51B is determined so asnot to affect the passing light, e.g., about 10 μm.

Accordingly, an aberration correcting element allowing accuratealignment can be provided by appropriately selecting the positions andshape of the position markers according to the shape of the pattern ofelectrodes of the aberration correcting element 41. Furthermore, compleximage processing is not necessary during alignment. Consequently,accurate alignment with the optical element can be easily made.

While an aberration correcting device applied to an optical pickup foran optical disc or the like has been described as an example, theinvention is not limited to this. The present invention can be appliedto devices for correcting aberrations in various optical systems. Inaddition, it should be noted that the numerical values given in theabove embodiments are merely exemplary. The above-described embodimentscan be utilized with appropriate modifications or in combinations.

As apparent from the foregoing, the present invention provides anaberration correcting element capable of being accurately aligned withan optical element, as well as a high-performance aberration correctingassembly in which an optical element and an aberration correctingelement are aligned with high accuracy.

The invention has been described with reference to the preferredembodiments thereof. It should be understood by those skilled in the artthat a variety of alterations and modifications may be made from theembodiments described above. It is therefore contemplated that theappended claims encompass all such alterations and modifications.

This application is based on Japanese Patent Application No. 2001-30513which is hereby incorporated by reference.

What is claimed is:
 1. An aberration correcting element, comprising: afirst electrode layer including a plurality of electrode portions; asecond electrode layer opposed to said first electrode layer; and aliquid crystal sandwiched between said first and second electrode layersfor producing a phase change in a light beam passing therethroughcorresponding to voltages applied to said first and second electrodelayers, wherein said first electrode layer includes a plurality ofposition markers.
 2. The aberration correcting element according toclaim 1, wherein said position markers are formed in at least threepredetermined positions that are not on a straight line.
 3. Theaberration correcting element according to claim 1, wherein saidposition markers are located close to an outer periphery of a regionwithin which said light beam is adjusted in phase.
 4. The aberrationcorrecting element according to claim 1, wherein said plurality ofelectrode portions are made of metal, and wherein said position markersare made of a metal that is identical to the metal of said electrodeportions.
 5. An aberration correcting assembly, comprising: a firstelectrode layer including a plurality of electrode portions and firstposition markers; a second electrode layer opposed to said firstelectrode layer; a liquid crystal sandwiched between said first andsecond electrode layers for producing a phase change in a light beampassing therethrough corresponding to voltages applied to said first andsecond electrode layers; and an optical element including secondposition markers corresponding to said first position markers.
 6. Theaberration correcting assembly according to claim 5, wherein said firstposition markers are formed close to an outer periphery of a pupil ofsaid optical element.